Oil pressure control system for an automatic transmission system

ABSTRACT

An oil pressure control system for an automatic transmission system which includes a fluid torque converter, a transmission gear and friction engaging means adapted to be selectively supplied with line pressure for establishing a selected transmission engagement in said transmission gear, said friction engaging means being provided with a buffering accumulator which buffers its operating pressure, said accumulator comprising a fluid displacing element which is selectively supplied with different levels of oil pressure, preferably a torque converter pressure or a line pressure as a back pressure according to the operational condition of the oil pressure control system.

BACKGROUND OF THE INVENTION

1. Field of the invention

The present invention relates to an oil pressure control system for anautomatic transmission system for use with automobiles and, moreparticularly, to an improvement in an oil supply system for a bufferingaccumulator incorporated in the oil pressure control system for reducingshock caused in speed shifting.

2. Description of the Prior Art

In an automatic transmission for use with automobiles which includes afluid torque converter, a transmission gear and friction engaging meansfor establishing a selected transmission engagement in said transmissiongear and is adapted to be controlled by an oil pressure control system,an accumulator is generally provided in an oil pressure circuit foroperating said friction engaging means in order to accomplish a smoothchanging over of the automatic transmission system by reducing the shockwhich is caused by the engagement of the friction engaging means. Anaccumulator for the abovementioned purpose conventionally comprises afluid displacing element such a cylinder-piston means wherein the pistonis flexibly supported by a spring means which counteracts the oilpressure existing in the oil pressure circuit for operating the frictionengaging means. Furthermore, it is known to supply an oil pressuregenerated in the oil pressure control system to said cylinder-pistonmeans as a back pressure for the piston in order to control thestiffness of the buffering accumulator. However, since a particularfriction engaging means such as the rear clutch is engaged in aplurality of operating conditions, i.e. D Range the highest speed(usually 3rd speed) condition as well as R Range, the back pressuresupplied to the cylinder-piston type buffering accumulator which isadjusted to provide favourable stiffness in an operating condition willnot be proper for the other operating condition. Generally, a higherstiffness of the buffering accumulator is favourable for R Range whencompared with D Range the highest speed shifting condition.Conventionally, the line pressure generated in the oil pressure controlsystem has been mostly employed as the back pressure for theaccumulator. Since the line pressure is usually raised or loweredaccording to increase or decrease of the torque to be transmitted by thetransmission system, it becomes higher in R Range than in D Range thehighest speed shifting condition. Therefore, this performance hasgenerally conformed to the requirement with regard to the stiffness ofthe buffering accumulator. However, for the modern oil pressure controlsystem, it is required that the stiffness of the buffering accumulatoris more delicately controlled as precisely adapted to the throttleopening so that the shock in the shifting be further reduced. To meetwith this requirement, it has been proposed to generate a particular oilpressure by a valve means for use as the back pressure for theaccumulator instead of the conventional line pressure. However, such asystem is still bound with the drawback that the back pressure is notadjusted for the change of the shifting condition like the changebetween R Range and D Range 3rd speed, thereby making it difficult toobtain favourable shock reducing performance in all shifting ranges.

SUMMARY OF THE INVENTION

Therefore, the object of the present invention is to provide an improvedoil pressure control system for an automatic transmission system for usewith automobiles wherein the buffering means for friction engaging meansis automatically adjusted for the most favourable shock absorbingcondition in every operational condition of the oil pressure controlsystem.

Other objects and further scope of applicability of the presentinvention will become apparent from the detailed description givenhereinafter; it should be understood, however, that the detaileddescription and specific examples, while indicating preferredembodiments of the invention, are given by way of illustration only,since various changes and modifications within the spirit and scope ofthe invention will become apparent to those skilled in the art from thisdetailed description.

According to the present invention, the above-mentioned object isaccomplished by providing an oil pressure control system for anautomatic transmission system which includes a fluid torque converter, atransmission gear and friction engaging means for establishing aselected transmission engagement in said transmission gear, comprising;a source of oil pressure; a line pressure regulating valve whichgenerates a regulated line pressure from the oil pressure of saidsource; a throttle pressure regulating valve which generates a throttlepressure from said line pressure, said throttle pressure increasing as athrottle valve is opened; a governor pressure regulating valve whichgenerates a governor pressure from said line pressure, said governorpressure increasing as the vehicle speed increases; a plurality of speedshift valves which are shifted due to a balance between said governorpressure and said throttle pressure so as to supply oil pressure to aselected element or elements of said friction engaging means; a manualshift valve which is operated by hand to supply oil pressure to aparticular element or elements of said friction engaging means and applya restriction to a selected one of said shift valves; a bufferingaccumulator for an element of said friction engaging means, saidaccumulator comprising a fluid displacing element which is supplied withoil pressure as a back pressure; a back pressure regulating valve whichgenerates a second oil pressure from said line pressure; and aswitch-over valve which selects one of several oil pressure sources insaid oil pressure control system according to the operational conditionof said system so as to supply oil pressure of a selected source to saidfluid displacing element.

BRIEF DESCRIPTION OF THE DRAWING

The present invention will become more fullly understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not limitativeof the present invention, and wherein,

FIG. 1 is a diagrammatical view showing an example of the automatictransmission system for use with automobiles which includes a fluidtorque converter, a transmission gear and friction engaging means forestablishing a selected transmission engagement in said transmissiongear;

FIG. 2 is a diagrammatical view showing the oil pressure control systemfor controlling the automatic transmission system shown in FIG. 1;

FIG. 3 is a graph which shows the performance of the line pressureobtained in the oil pressure control system shown in FIG. 2 in relationto the throttle opening; and,

FIG. 4 is a graph which shows the performance of the torque converterpressure obtained in the oil pressure control system shown in FIG. 2 inrelation to the throttle opening;

FIG. 5 is a graph which shows engine torque variance in relation to thethrottle opening.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, there is a diagrammatically shown automatictransmission system which comprises a fluid torque converter, atransmission gear and friction engaging means for establishing aselected transmission engagement in said transmission gear. Referencenumeral 1 designates the fluid torque converter which itself is wellknown, having an input shaft 2 and an output shaft 3. The input shaft 2is sometimes called a pump shaft and is directly connected to an outputshaft of an engine (not shown) or normally a crank shaft. The outputshaft 3 is sometimes called a turbine shaft and is directly connected toan input member of a front clutch 5 of a succeeding transmission gear 4.The output member of the front clutch 5 is carried by a firstintermediate shaft 6 which, in turn, carries a ring gear 7. The ringgear 7 meshes with rear planetary pinions 10 (only one is shown inFIG. 1) which are carried by a carried 9 supported by an output shaft 8of the transmission gear. The carrier 9 supports another ring gear 11which meshes with front planetary pinions 14 (only one is shown inFIG. 1) which are carried by a carrier 13 which in turn, is supported bya one-way clutch 12 to be rotatable only in one direction. Meshing withthe planetary pinions 10 and 14, sun gears 15a and 15b are supported bya second intermediate shaft 16 which is rotatable co-axially with saidfirst intermediate shaft 6. The driving member of the front clutch 5 isextended to form a driving member of a rear clutch 17, a driven memberof which is formed as a unitary body as said intermediate shaft 16. Therotation of the carrier 13 can be braked by a first-reverse brake 18.The rotation of the second intermediate shaft 16 can be braked by asecond-brake 19.

The transmission gear having the aforementioned constitution operates asfollows:

D Range, 1st speed . . . The front clutch 5 is engaged. Thus, therotation of the clutch (designated as clockwise rotation) is transmittedto the ring gear 7 through the first intermediate shaft 6, therebydriving the rear planetary pinions 10 clockwise. the sun gear 15a istherefore driven anticlockwise by the planetary pinions, applying, inturn, a rotational force to the front planetary pinions 14. Thus, thefront planetary pinions and the carrier 13 which carries the pinions areapplied with a rotational driving force in anticlockwise direction.However, since the rotation is prevented by the one-way clutch 12, thecarrier 9 is driven in clockwise direction, thus driving the outputshaft 8 in clockwise direction.

D range, 2nd sped . . . The front clutch is engaged, and the secondbrake 19 is actuated. The rotation of the turbine shaft 3 is transmittedto the ring gear 7 through the first intermediate shaft 6, therebydriving the rear planetary pinions 10 in clockwise direction. Since, inthis case, the sun gear 15a and the second intermediate shaft 16, bothbeing a unitary member, are restricted by the second brake 19, theplanetary pinions 10 are driven around the sun gear 15a, whereby thecarrier 9 which carries said planetary pinions is driven in clockwisedirection, thus driving the putput shaft 8 in the same direction.

D Range, 3rd speed . . . The front clutch 5 and the rear clutch 17 areengaged. The intermediate shaft 6 and the sun gears 15a,15b or theintermediate shaft 16 then rotate as a unitary body, rendering theplanetary gear mechanism inoperable. Therefore, the carrier 9 and theoutput shaft 8 rotate together with the turbine shaft 3 as a unitarybody without effecting any reduction in speed.

R (Reverse) range . . . The rear clutch 17 is engaged, and the reversebrake 18 is actuated. The clockwise rotation of the turbine shaft 3 istransmitted to the sun gear 15b through the intermediate shaft 16,applying a rotational driving force to the front planetary pinions 14 torotate same clockwise around the sun gear 15b. However, since therotation is prevented by the reverse brake 18, the front planetarypinions 14 are rotated anticlockwise around their axis, whereby thecarrier 9 and the output shaft 8 are rotated anticlockwise through thering gear 11.

L Range (locked at forward 1st speed) . . . The front clutch is engaged,and the the first-brake 18 is actuated. This range is the same D Range,1st speed in the engine drive condition, wherein the driving force istransmitted from the turbine shaft 3 to the output shaft 8. However, bycontrast to D Range, 1st speed, wherein the driving force cannot betransmitted from the ouput shaft 8 to the turbine shaft 3 as in theengine brake condition due to slippage of the one-way clutch 12, Rangeallows for the transmission of the driving force from the output shaft 8to the turbine shaft 3.

FIG. 2 is a diagram of an oil pressure control system incorporating thepresent invention for actuating the aforementioned front clutch 5, rearclutch 17, first-reverse brake 18 and second brake 19 in variouscombinations as mentioned above to accomplish a required operationalrange. In the figure, 20 designates an oil pump which generates an oilpressure which is delivered through a passage 21 to a port 23 of a linepressure regulating vavle 22. The valve 22 includes a valve member 25urged rightward in the figure by a coil spring 24 and another valvemember which abuts against end of the valve member 25. A line pressureis delivered from a port 27 of the valve 22, said line pressure beingregulated at a predetermined level by control pressures which areapplied to ports 28, 29 and 30 in a manner explained hereinunder. Theline pressure delivered from the port 27 is transmitted through apassage 31 to a port 33 of a throttle pressure regulating valve 32. Thevalve 32 comprises a valve member 36 maintained by the balance ofcompression coil springs 34 and 35 and another valve member 37 whichsuppports one end of the spring 34. The valve member 37 is applied witha leftward compression force at its right end according to the openingof a throttle valve of a carburetor (not shown) or the pressing of anaccelerating pedal. Thus, a port 38 of the regulating valve 32 deliversa throttle pressure which is modulated from the line pressure accordingto the throttle opening. On the other hand, the line pressure passesthrough the port 33 of the regulating valve 32 and is transmittedthrough a passage 39 to a port 41 of a manual control valve 40. Thevalve 40 comprises a valve member 42 which is shifted among positions P,R, N, D, 2 and L as shown in the figure by a change lever (not shown). Aport 43 of the manual control valve 40 is supplied with the linepressure when the valve member 42 is shifted to positions D, 2 and L,said line pressure being transmitted through a passage 44 to a governorpressure regulating valve 45. The regulating valve 45 generates agovernor pressure which increases according to the vehicle speed. Thegovernor pressure is transmitted through a passage 46 to left end ports49 and 50 of 1-2 speed shift valve 47 and 2.3 speed shift valve 48,respectively. The 1-2 speed shift valve 47 comprises a valve member 52urged leftward by a compression coil spring 51 and a valve member 53which supports a right end of the spring 51. The 2-3 speed shift valve48 comprises a valve member 55 urged leftward by a compression coilspring 54 and another valve member 56 which supports a right end of thespring 54. A port 57 of the 1-2 speed shift valve 47 and a port 58 ofthe 2-3 speed shift valve 48 are supplied with the throttle pressuredelivered from the port 38 of the throttle pressure regulating valve 32through a switching element 59 and a passage 60. The valve member 52 inthe 1-2 speed shift valve 47 is subjected to a balance between thegovernor pressure and the throttle pressure applied to the ports 49 and57, respectively, and is shifted rightward while compressng thecompression coil spring 51 when the governor pressure has risen beyond apredetermined level in relation to the throttle pressure. Similarly, thevalve member 55 in the 2-3 speed shift valve 48 is subjected to abalance between the governor pressure and the throttle pressure appliedto the port 50 and 58 respectively, and is shifted rightward whilecompressing the compression coil spring 54 when the governor pressurehas risen beyond a predetermined level in relation to the throttlepressure, said level being higher than the first mentioned predeterminedlevel which is determined for the 1-2 speed shift valve.

On the other hand, the line pressure delivered from the port 43 of themanual shift valve 40 is transmitted through a passage 61 to the frontclutch 5, while, simultaneously, it is transmitted through a passage 62to a port 64 of a detent pressure regulating valve 63 and, furthermore,it is transmitted through a passage 65 to a port 66 of the 1-2 speedshift valve. The line pressure supplied to the port 66 appears at a port67 of the speed shift valve when the valve member 52 is shiftedrightward and is transmitted through a passage 68 to a port 69 of the2-3 speed shift valve 48. The line pressure supplied to the port 69appears at a port 70 of the shift valve when the valve member 55 isshifted leftward as shown in the figure, and is transmitted through apassage 71 to a second brake 19. The line pressure supplied to the port69 of the 2-3 speed shift valve 488 appears at a port 72 of the speedshift valve when the valve member 55 is shifted rightward and istransmitted through a switching element 73 and a passsage 74 to the rearclutch 17.

The detent pressure regulating valve 63 generates a predetermined detentpressure at its port 75, said detent pressure being supplied to a port76 of the manual shift valve 40 wherefrom it is transmitted through apassage 77 to a port 78 of the throttle pressure regulating valve 32;through a port 79 and a passage 80 to ports 81 and 82 of the 2-3 speedshift valve 48 and/or through a port 83 and a passage 84 to a port 85 ofthe 1-2 speed shift valve 47 and, furthermore, through a passage 86 anda switching element 87 to a port 88 of the 1-2 speed shift valve. Theoil pressure supplied to the port 88 appears at a port 89 of the speedshift valve when the valve member 52 is shifted leftward as shown in thefigure, and is transmitted through a passage 90 to the first-reversebrake 18.

The manual shift valve 40 is provided with a reverse port 91. When thevalve member 42 is shifted to R (reverse) position, the line pressuresupplied to the port 41 of the manual shift valve appears at the port91, wherefrom the line pressure is transmitted through a passage 92 andthe switching element 87 to the port 88 of the 1-2 speed shift valve,and simultaneously, transmitted through a passage 93 to a port 94 of the2-3 speed shift valve. The line pressure supplied to the port 94 appearsat a port 95 of the shift valve member 55 is shifted leftward as shownin the figure and is transmitted through the switching element 73 andthe passage 74 to the rear clutch 17.

For the respective transmission ranges, the abovementioned oil pressurecontrol system operates as follows:

D Range . . . The manual shift valve 40 is shifted to D position,whereby its ports 43 and 76 are opened while the subsequent ports 79 and83 are still closed. Thus, the line pressure is transmitted from theport 43 through the passage 62 to the front clutch 5; through thepassage 44 to the governor pressure regulating valve 45; and through thepassage 62 to the port 64 of the detent pressure regulating valve 63and, furthermore, through the passage 65 to the port 66 of the 1-2 speedshift valve 47. In this condition, when the accelerating pedal ispressed to start the automobile, the throttle valve is opened, theengine gradually increases its output power, the automobile starts, andits speed gradually increases. Accordingly, the governor pressuregenerated by the governor pressure regulating valve 45 graduallyincreases. On the other hand, according to the opening of the throttlevalve, the valve member 37 of the throttle pressure regulating valve 32is applied with a certain level of leftward pressure and applies, inturn, an increased pressure to the valve member 36 via the spring 34,thereby causing an increase in the throttle pressure which appears atthe port 38. When the vehicle speed is below a predetermined level sothat the governor pressure generated in the passage 46 is below apredetermined level in relation to the throttle pressure supplied to thepassage 60, the valve member 52 of the 1-2 speed shift valve 47 and thevalve member 55 of the 2-3 speed shift valve 48 are both maintained attheir leftward shift positions as shwon in the figure. Therefore, thefront clutch 5 alone is engaged, establishing D Range, 1st speed. If thevehicle speed futher increases so that the governor pressure in thepassage 46 exceeds the aforementioned predetermined level, the valvemember 52 of the 1-2 speed shift valve 47 is shifted rightward. Then,the line pressure supplied to the port 66 is transmitted through theport 67 and the passage 68 to the port 69 of the 2-3 speed shift valve48, wherefrom it is transmitted through the port 70 and the passage 71to the second brake 19 to actuate same. In this condition, therefore, DRange, 2nd speed is accomplished. If the vehicle speed further increasesso that the governor pressure in the passage 46 increases beyond asecond predetermined level, the valve member 55 of the 2-3 speed shiftvalve 48 is shifted rightward. Then, the line pressure supplied to theport 69 of the shift valve is transmitted through the port 72, theswitching element 73 and the passage 74 to the rear clutch 17. In thiscondition, therefore, D Range, 3rd speed is accomplished. At this time,the oil pressure which has been supplied to the second brake 19 isdrained through a port 96 of the 2-3 speed shift valve. To the contrary,when the vehicle speed gradually decreases, the 2-3 speed shift valveand the 1-2 speed shift valve are successively shifted in the reversedorder. In this case, the 1-2 speed shift vale and the 2-3 speed shiftvalve are adapted to have a hysterisis regarding the balance of theshifting point oil pressure in order to provide stability in operationin the vicinity of the shifting point.

2 Range . . . The manual shift valve 40 is shifted to 2 position. Inthis condition, the ports 43, 76 and 79 of the manual shift valve areopened. By the port 79 being opened, the detent pressure delivered fromthe port 75 of the detent pressure regulating valve 63 is transmittedthrough the port 79 and the passage 80 to the ports 81 and 82 of the 2-3speed shift valve 48. Therefore, the valve member 56 is shifted leftwardand forcibly maintains the valve member 55 at the leftward shiftposition as shown in the figure. In this condition, therefore, shiftingto D Range, 3rd speed is prohibited and the transmission is operatedwithin the range of 1st speed and 2nd speed.

L Range . . . The manual shift valve 40 is shifted to L position. Inthis condition, the ports 43, 76, 79 and 83 are opened. Therefore, theline pressure is further transmitted through the port 83 and the passage84 to the port 85 of the 1-2 speed shift valve 47. Due to this oilpressure, the valve member 53 is shifted leftward and forcibly maintainsthe valve member 52 at the leftward shift position as shown in thefigure. Furthermore, the line pressure supplied to the passage 84 istransmitted through the passage 86 and the switching element 87 to theport 88, wherefrom it is transmitted through the port 89 and the passage90 to the first-reverse brake 18. Since at this time the valve member 55of the 2-3 speed shift valve 48 is also forcibly maintained at itsleftward shift position as shown in the figure, the front clutch 5 andthe first-reverse brake 18 are actuated thereby accomplishing L Range.

N Range . . . The manual shift valve 40 is shifted to N position asshown in the figure. In this condition, the line pressure does notappear at the port 43 and, accordingly, neither clutches nor brakes areactuated. The transmission gear of course does not transmit any drivingpower.

R Range . . . The manual shift valve 40 is shifted to R position. Theline pressure appears at the reverse port 91, wherefrom it istransmitted through the passage 93, ports 94 and 95 of the 2-3 speedshift valve 48, switching element 73 and the passage 74 to the rearclutch 17. Simultaneously, the line pressure is transmitted through thepassage 72, switching element 87, ports 88 and 89 of the 1-2 speed shiftvalve 47 and passage 90 to the first-reverse brake 18 to actuate same.

P Range . . . The manual shift valve 40 is shifted to P position. Inthis condition, the port 41 of the manual shift valve is blocked.Therefore, the line pressure is not transmitted to any succeedingportions in the oil pressure control system. The transmission gear ofcourse does not transmit any driving power.

The line pressure which is employed to accomplish the aforementionedvarious shifting operations is regulated by the line pressure regulatingvalve 22 in the below mentioned manner to be modulated according to theshifting ranges and the throttle opening. The port 29 of the linepressure regulating valve 22 is supplied with the throttle pressurewhich is delivered from the port 38 of the throttle pressure regulatingvalve 32 and transmitted through the passage 97. This throttle pressureacts to exert a rightward force to the valve member 25 of the linepressure regulating valve, thereby effecting an increase of the linepressure according to the opening of the throttle valve. The linepressure delivered from the port 27 of the line pressure regulatingvalve is transmitted through a passage 98 to a port 100 of a relay valve99. This relay valve comprises a valve member 102 urged leftward by aspring 101. When the valve member 102 is shifted leftward as shown inthe figure, the line pressure supplied to the port 100 is transmittedthrough a port 103 and a passage 104 to the port 28 of the line pressureregulating valve. The line pressure supplied to the port 28 acts to urgethe valve member 25 leftward, thereby effecting a reduction of the linepressure when compared with the condition wherein the line pressure isnot applied to the port 28. A port 105 of the relay valve 99 is suppliedwith the line pressure which appears at the port 67 of the 1-2 speedshift valve 47 through a passage 106. A port 107 of the relay valve issupplied with either the line pressure which appears at the port 64 ofthe detent pressure regulating valve 63 through a passage 108 and aswitching element 109 or the line pressure which appears at the reverseport 91 of the manual shift valve 40 through a passage 110, 111 and theswitching element 109. The port 30 of the line pressure regulating valve22 is supplied with the line pressure which appears at the reverse port91 of the manual shift valve through the passage 110. The line pressuresupplied to the port 30 acts to exert the valve member 26 rightward,thereby effecting an increase of the line pressure when compared withthe condition wherein the line pressure is not applied to the port 30.

Due to the abovementioned constitution, when the manual shift valve 40is shifted to R Range, the line pressure is transmitted from its reverseport 91 through the passage 110 to the port 30 of the line pressureregulating valve. At this time, the line pressure supplied through thepassage 110 is further transmitted through the passage 111 and theswitching element 109 to the port 107 of the relay valve 99. Since atthis time, the port 105 of the relay valve is not supplied with linepressure, the line pressure supplied to the port 107 shifts the valvemember 102 rightward, thereby intercepting connection between the ports100 and 103, whereby the port 28 of the line pressure regulating valveis not supplied with the line pressure. In this condition, therefore,the line pressure becomes the highest.

When the manual shift valve 40 is shifted to either of D, 2 and LRanges, and the shifting condition of the 1-2 speed shift valve and the2-3 speed shift valve is in the 1st speed condition, the line pressuredoes not appear at the reverse port 91 of the manual shift valve.Therefore, the port 30 of the line pressure regulating valve is notsupplied with the line pressure. Since in this condition the valvemember 52 of the 1-2 speed shift valve 47 is maintained at its leftwardshift position as shown in the figure, the line pressure supplied to itsport 66 is not transmitted to the port 67. The line pressure whichappears at the port 66 is transmitted through the passage 108 and theswitching element 109 to the port 107 of the relay valve 99, whereby thevalve member 102 of the relay valve is shifted rightward whilecompressing the spring 101 like in the aforementioned R Range andintercepts the connection between the ports 100 and 103. In thiscondition, therefore, the port 28 of the line pressure regulating valve22 is not supplied with the line pressure, resulting in a lower level ofthe line pressure when compared to that in the aforementioned R Range.

When the manual shift valve 40 is shifted to P, N, D, 2 or L Range, theline pressure does not appear at the reverse port 91 of the manual shiftvalve. Therefore, the port 30 of the line pressure regulating valve isnot supplied with the line pressure. Furthermore, when the manual shiftvalve is shifted to P or N range, the port 107 of the relay valve 99 isnot supplied with the line pressure. If the 2nd or 3rd speed range isattained with the valve member 52 of the 1-2 speed shift valve 47 beingshifted rightward, both of the ports 107 and 105 are supplied with theline pressure. Therefore, in either condition the valve member 102 ofthe relay valve is maintained at its leftward shift position as shown inthe figure by the action of the spring 101. Therefore, the port 28 ofthe line pressure regulating valve is supplied with the line pressure,effecting a further reduction of the line pressure when compared withthe aforementioned 1st speed condition.

The aforementioned performances of the line pressure regulated by theline pressure regulating valve are shown in FIG. 3.

Element 112 designates a back pressure or torque converter pressureregulating valve which comprises a valve member 114 urged rightward by acompression coil spring 113, a left end of said valve member beingsupported by a compression coil spring 115 which, in turn, is supportedby a cup-like valve member 116 at its left end. A port 117 of theregulating valve 112 is supplied with oil pressure which appears at arelief port 118 of the line pressure regulating valve 22 through apassage 119. By this arrangement, the pressure in the passage 119 isregulated by the regulating valve 112 at a second predetermined level,different from the line pressure, this second oil pressure being hereinreferred to as torque converter pressure. A port 120 of the regulatingvalve 112 is supplied with the throttle pressure which appears at theport 38 of the throttle pressure regulating valve 32, said throttlepressure acting to urge the valve member 114 rightward via the spring115 thereby effecting an increase of the oil pressure in the passage 119or the torque converter pressure according to increase of the throttlepressure. This increase of the torque converter pressure due to increaseof the throttle pressure is effected until the valve member 116 reachesthe terminal position in its rightward shifting. Thereafter, therightward pressure applied to the valve member 14 becomes constant,whereby the torque converter pressure generated in the passage 119becomes constant. The performance of the torque converter pressurecontrolled by the torque converter pressure regulating valve accordingto the throttle opening in the aforementioned manner is shown in FIG. 4.

The torque converter pressure generated in the passage 119 is suppliedthrough a passage 121 to the torque converter 1, wherefrom the oil ispassed through an oil cooler 122 to be effected cooling and, after beingpassed through lubricating portions 123, is returned to an oil tank 124.

The front clutch 5, the rear clutch 17 and the second brake 19 areprovided with accumulators 125, 126 and 127, respectively, to accomplishsmooth engaging operation by reducing shock caused by abrupt engagementor actuation thereof. The accumulators 125 and 127 for the front clutch5 and the second brake 19 are constantly supplied with the torqueconverter pressure through a passage 128 as their back pressure, whilethe accumulator 126 for the rear clutch 17 is supplied with either ofthe torque converter pressure in the passage 128 or the line pressureaccording to the switching-over operation of the switch-over element130, said line pressure being obtained from the passage 93 extendingfrom the reverse port 91 of the manual shift valve 40.

It is generally favourable that the buffering performance of theaccumulators or their stiffness increases as the engine torqueincreases. The engine torque varies according to the throttle opening ina manner as shown in FIG. 5. Therefore, comparing FIG. 5 with FIG. 4, itwill be noted that the manner of change of the engine torque in relationto the throttle opening resembles that of the change of the torqueconverter pressure in relation to the throttle opening. Therefore, theaccumulators 125 and 127 for the front clutch 5 and the second brake 19,which are supplied with the torque converter pressure as the backpressure are automatically controlled of their stiffness in a favourablemanner as adapted to the operation of the engine.

As for the accumulator 126 for the rear clutch 17, it is noted thatsince the line pressure does not appear at the reverse port 91 of themanual shift valve 40 in D Range, 3rd speed range, the accumulator 126is, in this range, supplied with the torque converter pressure from thepassage 128 through the switch-over element 130 and the passage 131,whereby the accumulator operates with a stiffness which is adapted forshifting to D Range, 3rd speed based upon the torque converter pressureas the back pressure. By contrast, when the manual shift valve 40 isshifted to R Range with its valve member 42 being set at R position, theline pressure appears at the reverse port 91, said line pressure beingintroduced into the switch-over element 130 through the passage 93.Them, the line pressure which is higher than the torque converterpressure effects switching-over of the switch-over element 130,interrupting supply of the torque converter pressure to the switch-overelement 130 while introducing itself through the switch-over element 130and the passage 131 to the accumulator 126. Thus, the accumulator 126 isnow supplied with back pressure which is higher than that supplied in DRange, 3rd speed condition thereby providing an increased stiffness forthe shock absorbing operation adapted for shifting to R Range.

From the foregoing, it will be appreciated that the accumulator whichoperates in different manner of establishing transmission in anautomobile transmission system is selectively supplied with differentlevels of oil pressure according to its operation conditions, therebyaccomplishing the most favourable shock absorbing performance in everyshifting condition.

We claim:
 1. An oil pressure control system for an automobiletransmission system which includes a fluid torque converter, atransmission gear and friction engaging means for establishing aselected transmission engagement in said transmission gear, comprising:asource of oil pressure; a line pressure regulating valve which generatesa regulated line pressure from the oil pressure of said source; athrottle pressure regulating valve which generates a throttle pressurefrom said line pressure, said throttle pressure increasing as a throttlevalve is opened; a governor pressure regulating valve which generates agovernor pressure from said line pressure, said governor pressureincreasing as the vehicle speed increases; a plurality of speed shiftvalves which are shifted due to a balance between said governor pressureand said throttle pressure so as to supply oil pressure to a selectedelement or elements in said friction engaging means; a manual shiftvalve which is operated by hand to supply oil pressure to a particularelement or elements of said friction engaging means and applying arestriction to a selected one of said speed shift valves; a bufferingaccumulator for an element of said friction engaging means, saidaccumulator comprising a fluid displacing element which is supplied withoil pressure as a back pressure; a back pressure regulating valve whichgenerates a second oil pressure from said line pressure, said secondpressure being lower than said line pressure and modified to vary inaccordance with said throttle pressure; and a switch-over valve adaptedto deliver an output pressure to said buffering accumulator, said outputpressure being a higher one of the two input pressures supplied thereto,wherein one of said two input pressures is said second pressure and theother of said two input pressures is either zero or said line pressurewhich appears at a port of said manual shift valve in accordance withthe shifting of said manual shift valve.
 2. An oil pressure controlsystem according to claim 1, wherein said switch-over valve is athree-ported ball check valve comprising a housing having a first portto be supplied with said line pressure, a second port to be suppliedwith said torque converter pressure and a third port connected to saidbuffering accumulator, and a ball mounted in said housing to be movablebetween first and second positions, said ball connecting said first andthird ports while isolating said third port from said second port whenit is located in said first position and connecting said second andthird ports while isolating said third port from said second port whenit is located in said second position.
 3. An oil pressure control systemaccording to claim 1, wherein said transmission gear includes aplurality of planetary gear mechanisms, each comprising planetary gearcomponents such as a sun gear, a ring gear, a plurality of planetarypinions and said friction engaging means includes a plurality ofclutches and brakes, comprising;a front clutch adapted to connect aninput power shaft to one component of said planetary gear mechanisms; arear clutch adapted to connect said input power shaft to a furthercomponent of said planetary gear mechanisms; a first-reverse brakeadapted to brake an additional component of said planetary gearmechanisms; and a second brake adapted to brake said further componentof said planetary gear mechanisms.
 4. An oil pressure control systemaccording to claim 3, wherein said transmission gear provides for threespeed forward drive transmissions and a reverse drive transmission; saidclutches include a front clutch, said rear clutch and said one-wayclutch, and said brakes include said first-reverse brake and said secondbrake, wherein said front clutch and said first-reverse brake and/orsaid one-way clutch are actuated for first speed transmission; saidfront clutch and said second brake are actuated for second speedtransmission; said front and rear clutches are actuated for third speedtransmission; and said rear clutch and said first-reverse brake areactuated for reverse drive transmission.
 5. An oil pressure controlsystem according to claim 4, wherein said clutch is provided with saidbuffering accumulator.
 6. An oil pressure control system according toclaim 5, wherein said front clutch and said second brake are providedwith other buffering accumulators, each comprising a fluid displacingelement which is constantly supplied with said second oil pressure asits back pressure.
 7. An oil pressure control system according to claim6, wherein said second oil pressure is also supplied to fluid torqueconverter as a torque converter pressure.
 8. An oil pressure controlsystem according to claim 4, wherein said switch-over valve selectseither said second pressure of the line pressure which appears at areverse port of said manual shift valve when said manual shift valve isshifted to R Range.
 9. An oil pressure control system according to claim1, wherein said back pressure regulating valve comprises a valve memberwhich controls a release port for said second oil pressure, said valvemember being basically positioned by a balance between said second oilpressure which acts to urge said valve member in a first direction toopen said release port and a spring force which acts to urge said valvemember in a second direction opposite to said first direction, saidvalve member being further applied with a second spring force which actsto urge said valve member in said second direction, said second springforce being applied with a compression spring which is supported by apiston member at its one end, said piston member being applied with thethrottle pressure which acts to bias said piston member in a directionto compress said compression spring for increasing said second springforce, a stroke of said biasing of said piston member being limited bythe piston member abutting against a positive stopper.
 10. An oilpressure control system according to claim 9, wherein said piston memberand said compression spring being designed so that the biasing stroke ofsaid piston member is fully traversed when the throttle pressure hasrised to a level which is attained by said throttle pressure regulatingvalve when the throttle valve is opened to an opening at which theengine torque substantually saturates.
 11. An oil pressure controlsystem according to claim 1, wherein said line pressure regulating valveprovides for three substantially different line pressure levelsaccording to the shifting conditions of said plurality of speed shiftvalves and said manual shift valve, the line pressure being at a highestlevel when said manual shift valve is shifted to R Range, anintermediate level when said manual shift valve is shifted to D, 2 or LRange and said speed shift valves are shifted to establish 1st speedtransmission, and a lowest level in other shifting conditions.
 12. Anoil pressure control system according to claim 11, wherein said linepressure regulating valve comprises a valve member which controls arelease port for said line pressure, said valve member being basicallypositioned by a balance between said line pressure which acts to urgesaid valve member in a first direction to open said release port and aspring force which acts to urge said valve member in a second directionopposite to said first direction, said valve member being furtherselectively applied with said line pressure in said first direction, thethrottle pressure in said second direction and said line pressure insaid second direction according to the shifting conditions of saidplurality of speed shift valves and said manual shift valve.
 13. An oilpressure control system according to claim 12, wherein said second linepressure applied to said valve member in said first direction iscontrolled by a relay valve which is controlled by the shifting of saidplurality of shift valves and said manual shift valve.
 14. An oilpressure control system according to claim 13, wherein said relay valvecomprises a valve member biased in a first direction by a compressionspring, said relay valve having two ports which communicate to eachother and transmit the line pressure which is applied to the valvemember of said line pressure regulating valve when the valve member ofsaid relay valve is biased in said first direction, the valve member ofsaid relay valve being selectively applied with the line pressure atopposite ends thereof in a manner that when the shifting condition ofsaid manual shift valve and said plurality of shift valve is establishedto obtain 1st speed transmission, the valve member of said relay valveis applied with the line pressure at its one end so as to be shifted ina second direction opposite to said first direction thereby interceptingtransmission of the line pressure through said two ports while in othershifting conditions, the valve member of said relay valve is either notapplied with any line pressure at opposite ends thereof or applied withthe line pressure at opposite ends thereof.
 15. An oil pressure controlsystem according to claim 14, wherein said line pressure which isselectively applied to the valve member of said line pressure regulatingvalve in said second direction is controlled by a reverse port of saidmanual shift valve so that said line pressure is applied when saidmanual shift valve is shifted to R Range.
 16. An oil pressure controlsystem according to claim 1, wherein said throttle pressure regulatingvalve comprises a valve member which controls a release port for theline pressure, said release port generating said throttle pressure, saidvalve member being basically positioned by a balance between thethrottle pressure which acts to urge said valve member in a firstdirection to close said release port and a spring force which acts tourge said valve member in a second direction opposite to said firstdirection, said spring force being adapted to be increased according toincrease of throttle valve opening.
 17. An oil pressure control systemaccording to claim 16, wherein said spring force is applied by acompression spring which is supported by a piston member at one endthereof, said piston member being displaced by a throttle valveoperating mechanism to compress said spring according to increase of thethrottle valve opening.
 18. An oil pressure control system according toclaim 3, wherein said clutches and brakes are operated by said linepressure, said line pressure being regulated by said line pressureregulating valve to be substantially different in three levels accordingto the shifting conditions of said plurality of shift valves and saidmanual shift valve, the line pressure in each one of said differentlevers being gradually modulated according to the level of said throttlepressure.